The present invention generally relates to a method for chemical mechanical polishing (CMP) of a photoresist layer
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and size of memory cells, and surface geometry of various features such as corners and edges.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The photoresist receives a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the photoresist. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the photoresist through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the photoresist as less soluble polymer.
The goal of the photoresist application process is to achieve a thin, uniform layer of photoresist on a wafer surface. Layers of photoresist materials are routinely applied to wafers multiple times during a manufacturing process for integrated circuits, as one of a sequence of steps to produce a desired lithographic pattern. The photoresist material is spin-coated onto to the wafer at a typical thickness ranging from 0.5 xcexcm to 2 xcexcm with better than 1% (1"sgr") non-uniformity. The photoresist generally may be described as a class of novolac/styrenic compounds, and are formulated currently to be used with either i-line (365 nm) or deep-ultraviolet (DUV, 248 nm or 193 nm) sources of illuminating energy.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching limits caused by the wavelengths of the optical radiation. A recognized way of reducing feature size of circuit elements is to lithographically image the features with radiation of a shorter wavelength. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (extreme ultraviolet (EUV), deep ultraviolet (DUV)), wavelength range of 5-200 nm are now at the forefront of research in an effort to achieve smaller feature sizes.
Although EUV lithography provides substantial advantages with respect to achieving high resolution patterning, the shorter wavelength radiation is highly absorbed by the photoresist material. Consequently, the penetration depth of the radiation into the photoresist is limited. The limited penetration depth of the shorter wavelength radiation requires use of ultra-thin photoresists so that the radiation can penetrate the entire depth of the photoresist in order to effect patterning thereof.
Photolithographic performance can be significantly enhanced via thinner photoresist layers. Thus, it would be desirable to controllably make thinner the photoresist layer at some point in the lithographic process after initial application of the photoresist to the wafer.
The present invention provides for a system and methodology for controllably reducing thickness of a photoresist layer. In particular, a planarization process is employed utilizing chemical mechanical polishing (CMP) techniques and a polishing liquid that allows for controlled removal of desired amounts of the photoresist layer. The present invention employs a polishing liquid which reacts with the photoresist (resin) at the surface/subsurface range. The degree of reaction is not great enough to cause rapid or measurable dissolution (e.g., chemical etching) of the photoresist, but reacts sufficiently to modify chemical bonding in the photoresist to facilitate surface layer removal by applied mechanical stress (e.g., via use of a CMP polishing pad).
The present invention through proper balance of mechanical abrasion and chemical reaction provides for controlled removal of photoresist to result in a substantially thin photoresist layer of desired, uniform thickness with a well-polished surface that is microscopically smooth and substantially free of surface defects and/or residues.
One aspect of the present invention relates to a system for controllably removing photoresist. A CMP system polishes the photoresist. A non-abrasive polishing liquid reacts with the photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by applied mechanical stress from the CMP system.
Another aspect of the present invention relates to a system for polishing a photoresist layer. A polishing pad is employed to polish the photoresist layer. A CMP drive system selectively applies the polishing pad against the photoresist layer at a predetermined downward force and rate of rotation. A measuring system is employed to facilitate measuring a thickness of the photoresist layer. A processor employs information from the measuring system to control the CMP drive system. A non-abrasive polishing liquid is used to react with the photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by applied mechanical stress from the CMP system.
Yet another aspect of the invention relates to a system for controllably removing photoresist, including: means for polishing the photoresist; and means for reacting with the photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by the means for polishing the photoresist.
Still yet another aspect of the subject invention relates to a method for controllably removing photoresist, including the steps of: using a CMP system to polish the photoresist; and using a non-abrasive polishing liquid adapted to react with the photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by applied mechanical stress from the CMP system.
Another aspect of the present invention relates to a polishing liquid for reacting with a photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by applied mechanical stress, including: at least one of KOH and (CH3)4NOH.
Still yet another aspect of the present invention relates to a method for polishing a photoresist layer. In the method, a polishing pad is used to polish the photoresist layer. A CMP drive system is used to selectively apply the polishing pad against the photoresist layer at a predetermined downward force and rate of rotation. A measuring system is employed to facilitate measuring a thickness of the photoresist layer. A processor utilizes information from the measuring system to control the CMP drive system. The method further includes the step of using a non-abrasive polishing liquid adapted to react with the photoresist to sufficiently modify bonding in the photoresist to facilitate surface layer removal of the photoresist by applied mechanical stress from the CMP system.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.